Passive diffusion. 3.2.1 Basic concepts Different transport routes have been proposed for passive penetration of molecules through the stratum corneum, a thin (10-20 μm), relative impermeable layer, which provides the rate limiting step for percutaneous penetration. Several macroroutes for penetration from the surface of the human skin into the subepidermal tissue are displayed in Figure 2 [5, 65, 68]: 1) via the sweat ducts 2) through the hair follicles with their associated sebaceous glands or 3) across the continuous stratum corneum between these appendages. It is generally believed that transport of most small lipophilic molecules occurs mostly via the latter pathway. This pathway can be subdivided into the transcellular route, though the corneocytes and lipid lamellae, and the intercellular pathway, solely through the lipid regions [65, 67]. For larger hydrophilic molecules and ions the appendegeal route may play also a significant role [68].
